This invention relates to the quantitative measurement of the amount of free oxygen in a gaseous mixture of the same with other gases. The invention finds application to the measurement of the precentage of oxygen content in fuel mixtures, both before and after burning, as in measurements in industrial combustion of fuels and in internal combustion engine analyses, and finds a special application in pulmonary function testing in which the inhalation and exhalation of a patient is being monitored for oxygen uptake.
The invention is based on the difference in magnetic susceptibility of oxygen gas, O2, (which designation will be hereinafter used in place of 0.sub.2) relative to other gases. It is well-known that oxygen gas is highly paramagnetic compared to most other gases and has a high positive value of magnetic susceptibility. Susceptibility is usually defined either by volume or by mass (volume divided by density) and especially relates to the property of a substance, (designated herein by k) which is a measure of the intensity of the magnetization induced in a unit volume of the substance by unit of applied magnetic field. A positive value indicates that the magnetic field within the substance exceeds the magnetic field in free space. The volume susceptibility will be referred to herein for the sake of uniformity and for oxygen gas is paramagnetic, positive and of the order of 10.sup.-7.
Oxygen gas is uniquely paramagnetic compared with other common gases which are usually weakly diamagnetic. Thus, the presence of oxygen can be quantitatively determined by the use of magnetic measurements of a sample gas compared to a reference gas of known content as for example; nitrogen free of oxygen (hereinafter N2), oxygen in a pure form, or a known mixture of oxygen and gases such as ambient air.
For some time a need has existed for an instrument and system capable of accurately measuring the free gaseous oxygen content of sample gases. Such an instrument should be accurate and it should respond rapidly. Attempts to use the difference in magnetic properties of oxygen compared to other gases have been proposed and generally rely on absolute magnetic flux measurements or differences in the absolute magnetic flux which can be obtained in substituting sample and reference gas mixtures in magnetic circuits. However, the susceptibility difference in such systems amounts to such a small part of the entire measurement that the measurement technique required a remarkable insensitivity to any other factors which are present which can be at least as large or larger than the changes in oxygen content. Of these, thermal effects, slight changes of the measuring geometry caused by mechanical movements, stray electric fields and magnetic fields all cause changes in the measuring instrument which are often of a magnitude which destroy the possibility of successfully making the measurement required. While many systems have been proposed, it is not believed that any have achieved freedom from interference and sensitivity requisite of a good practical instrument.
Examples of the foregoing include U.S. Pat. No. 3,049,665 to Hummel in which a sample and reference cells enclosed within sensing coils are formed in a two-arm magnetic circuit by having a common element including a driving coil. The absolute values of the induced fields in the arms are sensed by the coils to produce electrical signals which are combined to produce a difference signal ostensibly proportional to the magnetic susceptibility of the sample and reference gases. This is in essence an electrical signal subtraction of signals derived from changes in the entire magnetic field of magnetic circuits including sample and reference gas gaps, but is found to be impractical because the thermal, mechanical, and magnetic disturbances are at least as large as the difference sought to be measured and mask the effect.
U.S. Pat. No. 2,467,211 to Hornfeck also uses absolute difference measurements in the reluctance of a flux path caused by alternating the sample and reference gases through the path. This also requires detecting changes of the order of 10.sup.-7 of the total effect measured and is subject to the same difficulties as the Hummel patent, '665.
U.S. Pat. No. 2,689,332 to Green proposes an improved structure for balancing out mechanical vibrations and would appear to be effective but nevertheless requires the measurements of the gross change in the magnetic flux caused by a change in the susceptibility of the sample in a gap. U.S. Pat. No. 3,720,870 to Swatha proposes changing the sample and reference gases through a gap in a single magnetically biased circuit and noting the changes in induced voltages in the coil due to the difference in magnitude due to susceptibility. This still requires measurement of the overall gross magnetic flux due to the first and second samples in order to obtain a difference. In sum, the references disclosed rely on the ability of a electrically connected coil to sense a change in a very large value of total flux density and thus are inherently incapable of making a direct measurement of the flux difference while excluding the presence of the gross magnetic flux from the measurement circuit and are thus subject to the common difficulty of attempting to measure in the presence of a major flux a change of extraordinarily small magnitude.
There is therefore, a need for a new and improved method and apparatus for analyzing gases for oxygen content and for determining the percentage of oxygen quantitatively.